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Soy isoflavones can be extremely useful in forestalling and relieving osteoporosis after menopause. But there is a catch.

The best way to describe the benefits of soy products for bone health is this advice for older women who have osteroporosis is this:

"Eat soy. It's good for your bones but...."

Many studies confirm that soy isoflavones are useful for treating osteporosis in women who have passed menopause. But many other studies do not. The problem with the study may be that soy isoflavones have to be activated by probiotic bacteria in the intestines before they are able to stimulate the growth of new bone. When gut health is poor, and the number of friendly bacteria in the gut is minimal, soy isoflavones are unable to activate formation of new bone. But if you have established good gut health, soy isoflavones may be very helpful in maintaining healthy bones.

How do we know the benefits of soy in women's bone health?

A group of researchers at the Department of Internal Medicine at the  University of Messina in Italy recruited 90 healthy women aged 47 to 54. They gave them a battery of tests, including DXA scans of bone mineral density, and then put them on estrogen replacement therapy, soy isoflavones (this study used genistein), or a placebo for a year. After twelve months on either soy or estrogen treatment or a placebo, the results were:

  • The placebo left serum levels of bone‐specific alkaline phosphatase and osteocalcin unchanged. Bone-specific alkaline phosphatase is an indicator of the earliest signs of bone-building activity. Osteocalcin is a hormone that stimulates the mineralization of bone.
  • Estrogen replacement therapy decreased these two measurements. That is, estrogen somehow suppressed the earliest stages of bone building activity and stopped the mineralization of bone.
  • Soy isoflavones increased both bone-specific alkaline phosphatase and osteocalcin after six months of treatment.

Both soy isoflavones and estrogen increased bone mineral density. Soy had a greater mineralization effect in the neck of the femur. Estrogen replacement therapy had a greater mineralization effect in the lumbar spine. No woman on either treatment lost bone mineral mass, although some gained less mineralization than others. Both soy and estrogen increased mineralization about 3 percent.

Why would anyone prefer soy isoflavones to estrogen replacement therapy?

Both estrogen replacement therapy and soy isoflavones control some of the other uncomfortable symptoms of menopause, but they do not control all symptoms equally well. In the study above:

  • Soy isoflavones did a better job of controlling vaginal bleeding and breast tenderness, but
  • Estrogen replacement therapy (ERT) did a better job of controlling vaginal dryness and hot flushes.

Women who have a particular difficulty with these symptoms of menopause my choose one treatment over the other on that basis. But for most women, the decision will be made on the basis of side effects of estrogen replacement therapy that aren't known with soy isoflavones. These include hypertension, high cholesterol, and gallbladder disease, and well as increased risk of breast and endometrial cancers. Soy does not increase the risk of cancer, and it increases the formation of healthy bone while it also controls symptoms of menopause. It is also much less expensive than estrogen replacement therapy. For most women, the choice is soy. However, there are some things women need to know about soy in order to use it effectively.

How to use soy for breast health?

It is not necessary to eat soy at every meal in order to get soy isoflavones. While you certainly can soy for every meal if you desire, most women will find it easier to take soy isoflavones. There is one specific isoflavone that is proven to enhance bone health. That compound is genistein. If the product does not contain genistein, it probably is not going to be helpful for bone health. 

Another reason to use isoflavone supplements rather than eating soy several times a day is that the body's ability to absorb the isoflavones from food is limited. No matter how much soy a woman eats, her body can only process about 15 mg of genistein in any 24-hour period. Soy isoflavone supplements provide 50 to 100 mg of genistein in a form relatively easy to absorb into the body.

Rather than load up on soy to get your isoflavones, be sure to eat a variety of foods that nourish the bacteria that help your body absorb genistein. These foods include soy (for your bacteria, not just for you), rye bread, many berries, alfalfa sprouts, clover sprouts, broccoli, cauliflower, and sunflower seeds, Soybean oil and soy sauce contain almost no genistein.

But for best results, combine soy isoflavones with probiotic supplements.

Soy isoflavones are more effective when they are taken about six hours after taking a probiotic supplement. This is especially important for women who have had to take antibiotics. (If you are on antibiotics, you may do better to pause your isoflavone supplement until you finish the antibiotics.) Take the probiotic with breakfast and your isoflavones at noon or in the afternoon for maximum absorption of genistein and maximum benefit to your bones.

  • Clavel T, Fallani M, Lepage P, Levenez F, Mathey J, Rochet V, Sérézat M, Sutren M, Henderson G, Bennetau-Pelissero C, Tondu F, Blaut M, Doré J, Coxam V. Isoflavones and functional foods alter the dominant intestinal microbiota in postmenopausal women. J Nutr. 2005 Dec.135(12):2786-92.PMID: 16317121.
  • Morabito N, Crisafulli A, Vergara C, Gaudio A, Lasco A, Frisina N, D'Anna R, Corrado F, Pizzoleo MA, Cincotta M, Altavilla D, Ientile R, Squadrito F. Effects of genistein and hormone-replacement therapy on bone loss in early postmenopausal women: a randomized double-blind placebo-controlled study. J Bone Miner Res. 2002 Oct.17(10):1904-12. PMID: 12369794.
  • Xu X, Duncan AM, Merz BE, Kurzer MS. Effects of soy isoflavones on estrogen and phytoestrogen metabolism in premenopausal women. Cancer Epidemiol Biomark Prev 7: 1998. 1111–1118.
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